In response to a trend in the electronics industry toward increased functionality and miniaturization, the number and complexity of integrated circuits required is increasing while the amount of area available to receive integrated circuit packages on a printed circuit board substrate is decreasing. Integrated circuits, microprocessors in particular, have an increasing array of functionality, have greater numbers of input/output("I/O") ports and are running at higher clock rates. Microprocessors implement some of their functionality through use of cache memory. The speed in which microprocessors perform a certain functions is related to the time required to access cache. There is limited cache memory directly on microprocessor chips where access time is at a minimum. In certain cases, however, some functions performed by microprocessors, require access to greater blocks of cache than is available directly on the microprocessor chip. Rather than provide very large blocks of memory directly on the microprocessor chip, those microprocessor functions requiring a large block of cache use memory remote of the microprocessor. As the speed of the function is inversely related to the access time, it is important to minimize the access time to the remote memory and desirable to have as much cache memory available as possible. One way to minimize the access time is to minimize the electrical length of the connection between the I/O ports on the microprocessor, or other integrated circuit, and the I/O ports on memory to which the microprocessor communicates. One method of increasing memory capacity and decreasing both electrical length and physical size is to mount multiple integrated circuit memory chips onto a single substrate. This type of assembly is typically termed a multichip module. Multichip modules minimize excess packaging, excess packaging being associated with increased electrical length. Therefore, there is a need to socket memory modules as closely as possible to the microprocessor or other circuitry that accesses them.
In keeping with the goal of miniaturization, these sockets must take up a minimum amount of area on a printed circuit board substrate. There are many different types of low profile sockets such as the one disclosed in U.S. Pat. application Ser. No. 08/075,698 that discloses a low profile integrated circuit socket. Low profiles are important in cases where many printed circuit boards are stacked closely together creating limited clearance from board to board. Under certain circumstances, however, it is less important for a socket to have a low profile, but crucial that the socket have a small footprint. In addition, a standard industry requirement is that the sockets and corresponding multichip modules be able to withstand at least 100 Gs of physical shock without experiencing an electrical discontinuity greater than one microsecond in duration. There is a need, therefore, for an integrated circuit socket having a small footprint and a short electrical length capable of withstanding 100 Gs of physical shock without experiencing significant electrical discontinuity.
Multichip module substrates may be made of, among other materials, ceramic, aluminum and laminates. Conductive traces on the substrate make an electrical connection between I/O ports on the multichip modules and leads on the edge of the substrate. The leads may be on a single side of the substrate on 0.0125 inch centerline spacings. Alternatively, half of the leads may extend to an edge on one side of the substrate, and the remaining half of the leads may extend to the opposite side in a double sided substrate. In the double sided substrate, leads may be on 0.025 inch centerline spacings. It is the size of the substrate, the number of leads on the substrate, and the placement of the leads that dictate the appropriate number of leads and the substrate configuration, both of which can vary widely. Certain socket applications tend to be relatively low volume rendering it difficult for a socket manufacturer to offer low cost through economies of scale. In a competitive market environment where time to market is of the essence and the supplier with the lowest cost has a competitive advantage, it is important to be able to respond to industry needs quickly and at a minimum cost. There is a need, therefore, for a manufacturable multichip module socket design applicable to both single sided and double sided substrates of varying lengths.
The advent of surface mount solder processing contributed to higher density industry applications due to the capability to achieve smaller centerline spacings for adjacent solder contacts. The most common current industry capability for surface mount soldering at acceptable yields is on 0.025 inch centerline spacing. It is advantageous, therefore, to have a socket that is able to interface 0.0125 inch centerline spacing of integrated circuit substrates with 0.025 inch centerline spacing of current solder processing capabilities. Furthermore, it is expected that solder processing capabilities will improve in the relatively near future as technology progresses. There is, therefore, a need for a high density socket having a footprint compatible with current processing capabilities and adaptable to next generation processing capability.